To comply with legally required emission limits, modern motor vehicles are fitted with a tank ventilation device. The fuel vapors produced in the fuel tank are fed to an activated carbon filter where they are adsorbed. However, as the storage capacity of the activated carbon canister is limited, it must be regenerated from time to time. To this end, the activated carbon canister is connected to the intake manifold of the internal combustion engine via a venting line and a tank vent valve disposed therein. To regenerate the activated carbon canister, the tank vent valve is opened causing the fuel vapors absorbed in the activated carbon canister to be sucked into the intake tract of the internal combustion engine because of the negative pressure in the intake manifold and to participate in combustion as part of the fuel/air mixture. As a result of this tank venting process or rather regeneration process, an initially unknown amount of hydrocarbons is supplied to the engine so that the composition of the combustible mixture changes. As each change in the composition of the combustible mixture directly affects the combustion process and the exhaust gas composition of the engine, precise control of the tank vent valve is necessary.
The tank vent valve is mostly an electromagnetic valve whose degree of opening is set by means of a pulse-width-modulated control signal (PWM signal). For precise execution of the tank venting process it is necessary to know the opening instant of the tank vent valve, i.e. the control signal value at which the passage of gas through the tank vent valve occurs. This opening control value may vary because of manufacturing tolerances, fouling, deposits and other changes over service life.
According to a known strategy for estimating the opening control value, the tank vent valve is partially opened and the output signal of a lambda controller device of the internal combustion engine is monitored. As soon as the tank vent valve opens, the exhaust gas composition changes because of the additionally supplied hydrocarbons, which is detected by the lambda controller device. Consequently, as soon as a change in the output signal of the lambda controller device occurs, the opening control value of the tank vent valve can be determined. However, this method is subject to considerable limitations. In order to obtain a sufficient deviation of the lambda controller signal, the activated carbon canister must have a high degree of loading. Moreover, the method can only be carried out when the intake manifold pressure is low enough to suck in the fuel vapors. Particularly in the case of supercharged engines or engines with load control via the valve stroke, these conditions are only rarely encountered. In addition, this method is imprecise.